Transistor having a relatively high inverse alpha



May 24, 1966 l I` HAAS TRANSISTOR HAVING A RELATIVELY HIGH INVERSE ALPHA ATTORNEYS May 24, 1966 l. HAAS TRANSISTOR HAVING A .RELATIVELY HIGH INVERSE ALPHA Filed June 2l, 1962 Z Sheets-SheeiI 2 D F/G/` r v l Knq- Lp F/G./7

' INVENToR.

lsY HAAs ATTORNEYS United States Patent O 3,253,197 TRANSISTOR HAVING A RELATIVELY HIGH IN VERSE ALPHA Isy Haas, Cupertino, Calif., assignor to Amelco, Inc., Los Angeles, Calif., a corporation of California Filed June 21, 1962, Ser. No. 204,160 8 Claims. (Cl. 317-235) This invention relates generally to a transistor, and more particularly to a transistor having high inverse alpha.

To maximize the alpha or transport factor of junction transistors, they are generally manufactured with a collector junction which is larger in size than the emitter junction to assure that substantially all the minority carriers which diffuse through the base layer are collected by the collector even if some spreading occurs. When transistors of this type are operated in a reverse connection, i.e., with the collector serving as emitter and the emitter as collector, the inverse alpha is relatively poor since a substantial number of minority carriers do not reach the collector.

Furthermore, in order -to obtain a high injection efficiency, which is necessary for a high alpha, the doping of the emitter must be much higher than the base. Since this is not the case in a conventional diffused transistor, this also contributes to a low inverse alpha.

Where the switching circuits are formed by microcircuit techniques, it is desirable to be able to construct a plurality of devices on a single Wafer without having to undergo elaborate measures for isolating and interconnecting the devices. Conventional transistors require isolation and ohmic interconnection. The reverse connection with common emitters would permit the formation of circuits having a number of transistors on a single wafer.

It is an object of the present invention to provide a transistor having a high inverse alpha.

It is another object of the present invention to provide a transistor structure which can be incorporated in integrated microcircuits having common emitter connections without requiring elaborate isolation or interconnection.

It is another object of the present invention Ito provide a transistor having a relatively high injection efficiency when operated in the inverse configuration.

It is another object of the present invention to provide a transistor which can be operated in the inverse configuration with minimum overlapping diode effect.

These and other objects of the invention will become more clearly apparent from the following descrip-tion when taken in conjunction with the accompanying drawing.

Referring to the drawing:

FIGURE l shows a sectional view of a prior art transistor;

FIGURE 2 shows a transistor having a high inverse alpha;

FIGURE 3 shows another transistor having high inverse alpha;

FIGURE 4 shows a plurality of transistors of the type shown in FIGURE 3 used in lan integrated microcircuit;

FIGURE 5 shows still another transistor in accordance with the invention;

FIGURES 6-11 show the steps in forming a transistor of the type shown in FIGURE 3; and

FIGURES 12-17 show a method of forming a transistor of the type shown in FIGURE 5.

Referring to FIGURE 1, a conventional transistor is illustrated. The transistor includes an n-type body forming the collector region. Ohmic contact is made to the collective region and a collector lead c is attached thereto. Inset into the n-type body is a p-type base region. Ohmic 3,253,197 Patented May 24, 1966 ICC contact is made to -the base region and base lead b is connected thereto. Inset into the p-type base region is an n-type emitter region. Ohmic contact is made to the emitter region and emitter lead e is connected thereto. Typical impurity thickness, concentrations and resistivities for the various regions are as follows:

Base Depth-l.8 microns Impurity concentration-5x1018 boron atoms per cm.3 Sheet resistance: V/ I 15 ohms/ square Emtter Depth-1.5 microns Impurity concentration-4021 phosphorus atoms per cm.3 Sheet resistance: V/l=0.6 ohm/square Collector impurity concentration-40154016 atoms phosphorus per cm.3 Sheet resistance :V/Il ohm/ square.

Carriers injected at the emitter junction diffuse through the relatively thin portion 11 of the base region and are collected by the relatively large collector junction 12. Any carriers which spread outwardly from the emitter junction 14, as indicated by the dotted lines 13 which are representative of the average lifetime, are also collected by the collector junction. -The transport factor (alpha) of the transistor is relatively high.

However, when a transistor of the foregoing character is operated in the reverse connection, that is, the collector junction is forward biased to act as the emitter to inject carriers into the base region and the emitter junction 14 is reverse biased to serve as the collector to collect the injected carriers, lthe alpha is relatively low since carriers not only tend to diffuse towards the collector region but into the remainder of the base region where they may recombine.

Referring to FIGURE 2, there is shown a transistor in which the inverse transport factor (alpha) is relatively high. The transistor illustrated includes an n-type emitter region inset -into an inse-t p-type base region. However, the inset regions are inset into an epitaxially grown n-type layer which is grown onto an n-lwafer which has a ring-like region of low impurity concentration nwhich leaves an n-jcolumn disposed opposite the emitter region. The 11+ column is made of substantially the same configuration and area as the emitter region. The column preferably has a higher impurity concentration than the collector region forming the basecollector region. For example, it may have an impurity concentration of 1.2 1021 phosphorus atoms per cm.3 giving a sheet resistance of 0.5 ohm/square, while the ring may have an impurity concentration of 1015 phosphorus atoms per cm.3 giving a resistivity of 5 ohms cm.

Operation of the transistor with conventional connection is substantially identical to that described above. In the reverse connection, most of the carriers are injected by the portion 12a of the emitter junction which overlies the n-jcolumn. This is due to the fact that the n{ column has a higher conductivity whereby the voltage drop through the column is relatively low in comparison to the remainder of the collector region to give a higher voltage at that portion of' the junction 12a adjacent the column than at the other portions 12b `of the collector junction overlying the nportions of the region. In the inverse configuration, the active portion of the emitter is substantially the same area as the collector which results in a relatively high alpha value.

In the configuration, however, there is still some loss of injection eiciency because of the forward bias applied to the other portions 12b of the emitter junction. This can be overcome to some extent by a device of the type shown in FIGURES which, in place of the nregion or ring surrounding the column, there is provided a p-type region or ring which leaves a relatively thin n-type region 18 adjacent the other portions 12b of the junction. The resistance drops along this thin region are such as to reduce the bias on the adjacent portions 12b of the emitter junction to reduce the injection in this portion of the junction.

Referring to FIGURE 4, a plurality of devices of the type shown in FIGURE 3` are formed on a single wafer. The inverse emitter connection er is made to the body. Due to the higher conductivity in the n-jcolumns, the device will operate at the junction portions 12a. Thel p-type ring surrounding the n| column will serve to reduce any injection in the regions of the junction 12b which surround the operating regions.

. Referring to FIGURE 5, there' is shown a device in which the n-lcolumn is formed in a p-type body to thereby further improve the injection efiiciency.

Referring to FIGURES 6-ll, there are illustrated the steps in forming a transistor of the type shown in FIG- URE 3. A p-type wafer having the desired impurity concentration is selected, FIGURE 6. Subsequently, one surface of the wafer is masked leaving a Window through which n-{ regions may be'diffused. An n-type diffusion is carried out until the diffusion layer from the other surface meets the diffusion layer formed at the window as shown in FIGURE 7. Subsequently, an epitaxial n-type layer is grown on one surface of the wafer, FIGURE 8. A subsequent masking operation leaves a window through which the base layer is diffused. The vdiffusion forms the inset base layer, FIGURE 9. Another masking operation then forms a smaller window through which the emitter n-type region is diffused, FIGURE l0. The nal step is application of ohrnic contacts to the various regions and the attachment of terminal leads, FIGURE ll.

The steps in forming a device in accordance with FIG- URE are illustrated in FIGURES 12-17 and comprise selecting a p-type wafer having the desired impurity concentration, FIGURE l2. By masking to provide opposed upper and lower windows in the mask and a subsequent diffusion, an n-jcolumn is formed in the wafer, FIGURE 13. Subsequently, an epitaxial n-type layer is grown on one surface of the wafer, FIGURE 14. The epitaxial layer is then masked and an inset p-type base region is formed therein, FIGURE l5. Subsequently, masking and diffusion operations provide an n-type emitter region, FIGURE 16. Ohmic contacts are then provided as shown in FIGURE 17. v

The transistor shown in FIGURE 2 may be formed by either maskingV and diffusing into a low doped n-type wafer to form the highly doped column or by starting with a highly doped block and masking and out diffusing to form the l'ow doped ring. Subsequent steps may be epitaxial growth followed by masking and diffusion steps to form inset base and emitter regions.

The structure shown in FIGURE 4 may belformed similarly to that shown in FIGURE 3 providing, however, masks which have a plurality of windows lproperly registered during each operation.

Thus, it is seen that there is provided a transistor which has relatively high inverse alpha and a method of making the same. The transistor can be incorporated in integrated microcircuits having a common emitter configuration.

I claim:

1. A transistor comprising a collector region, a base region forming a base collector junction therewith, and an emitter region forming an emitter base junction of predetermined configuration and area with the base region in which said collector region includes a first portion having aY first impurity concentration forming with the base region a base-collector junction, and a second portion of higher impurity concentration spaced from the base-collector junction opposite the emitter region, said second collector portion having substantially the same area and configuration as the emitter region.

2. A transistor as in claim 1 in which said higher impurity concentration portion. is surrounded by a region of low impurity concentration.

3. A transistor as in claim 1 wherein said high irnpurity concentration portion is surrounded by a region of opposite conductivity type.

4. A transistor as in claim 1 wherein said high impurit concentration portion of the collector is a column inset in the collector region.

5. A transistor-comprising a collector region, a base region inset into said collector region and forming a base collector junction therewith, an emitter region inset into said base region and formingy an emitter base junction of predetermined configuration and area therewith, in which said collector region includes a portion having a first impurity concentration forming a junction with the base region and a portion of high impurity concentration spaced from said junction presenting aface to the emitter of substantially the same area and configuration as the emitter.

6. A transistor comprising contiguous collector, base and emitter regions defining emitter-base and base-collector junctions in which said emitter-base junction has an area substantially less than the-base-collector junction, and said collector region includes a portion opposite the emitter-base junction having a greater conductivity than any other portions of the collector region to give a high voltage at the portion of the collector-base junction lying opposite the emitter-base junction when the transistor is operated in reverse connection.

7. A transistor as in claim 6 wherein said portion cornprises a column.

8. A transistor comprising contiguous collector, base and emitter regions defining emitter-base and base-collector junctions in which said emitter-base junction has a predetermined configuration and area substantially less than that of the base-collector junction, and said collector region includes a portion of material having a higher conductivity than the remainder of the collector, said portion having substantially the same cross-sectional area.

No references cited.

JOHN w. HUCKERT, Primary Examiner.

I, D. KALLAM, C. E. PUGH, Assistant; Examiners, 

1. A TRANSISTOR COMPRISING A COLLECTOR REGION, A BASE REGION FORMING A BASE COLLECTOR JUNCTION THEREWITH, AND AN EMITTER REGION FORMING AN EMITTER BASE JUNCTION OF PREDETERMINED CONFIGURATION AND AREA WITH THE BASE REGION IN WHICH SAID COLLECTOR REGION INCLUDES A FIRST PORTION HAVING A FIRST IMPURITY CONCENTRATION FORMING WITH THE BASE REGION A BASE-COLLECTOR JUNCTION, AND A SECOND PORTION OF HIGHER IMPURITY CONCENTRATION SPACED FROM THE BASE-COLLECTOR JUNCTION OPPOSITE THE EMITTER REGION, SAID SECOND COLLECTOR PORTION HAVING SUBSTANTIALLY THE SAME AREA AND CONFIGURATION AS THE EMITTER REGION. 